Moved from Mosquitoes.

I just realized that your pupa went to larva instead of the other way around. Interesting. But I may have an answer. I wonder if they treat your water with any larvacide?

Anyway, there is a larvacide called methoprene. The main company that manufactures it, manufactures it under the name Altosid. The active ingredient in the larvacide is juvenile hormone. Juvenile hormone is an important hormone that keeps an insect as a 'juvenile' (larva) and then when its production slows down it becomes an adult. So the mosquito control people put this in water so mosquito larvae never become adults. They usually die as pupa. But there is a chance your larva got an extra dose of methoprene and swtiched its phase. Do you know if it died when it became a larva?

indicates a culicine larva. and a pair of ventral setae indicates a Culiseta

now we're really confused!